1. Field of the Invention
The present invention relates to a surface acoustic wave actuator and a deflector employing the same.
2. Description of Related Art
Surface acoustic wave actuators generally demonstrate excellent working performance such as high speed, high thrust, and high resolution, and due to this, they are actively researched and developed for linear actuators, etc.
FIGS. 1A and 1B are perspective views explaining a surface acoustic wave actuator according to a related art 1, in which FIG. 1A schematically shows the structure of the surface acoustic wave actuator (or motor) and FIG. 1B shows a concept of frictional drive by Rayleigh waves.
In FIG. 1A, the surface acoustic wave actuator 100 of the related art 1 is developed as, for example, a surface acoustic wave motor. The surface acoustic wave actuator 100 has a piezoelectric board 101 made of, for example, lithium niobate and having a rectangular shape. At right and left ends on a top surface 101a of the piezoelectric board 101, comb-shaped electrodes 102A and 102B are formed by, for example, vapor deposition. The comb-shaped electrodes 102A and 102B are connected to high-frequency power sources 104A and 104B through switches 103A and 103B, respectively. On the top surface 101a of the piezoelectric board 101, a slider 105 which is preloaded is movably arranged between the comb-shaped electrodes 102A and 102B.
If the right switch 103A, for example, is turned on, a high frequency is supplied from the high-frequency power source 104A to the comb-shaped electrode 102A. The high frequency converts to Rayleigh waves LW that advance from the right side toward the left side in the direction of an arrow mark X2 on the piezoelectric board 101. The Rayleigh waves LW propagate along the piezoelectric board 101 to move the preloaded slider 105 against the Rayleigh waves LW in the direction of an arrow mark X1. At this time, the left switch 103B is kept OFF.
In FIG. 1B, Rayleigh waves LW cause surface particles of the piezoelectric substrate 101 to rotate in elliptic loci, to thereby move the slider 105 that is in contact with the heads of the Rayleigh waves LW in the direction X1 by frictional drive. At this time, some load must be applied to the slider 105 from above the same to produce a sufficient friction. Without such friction, the slider 105 may simply oscillate up and down. Accordingly, the slider 105 is preloaded with, for example, the own weight thereof or with a spring (not shown).
If the piezoelectric board 101 with the preloaded slider 105 is inclined or is installed upside down, the slider 105 will drop off the piezoelectric board 101. Therefore, the piezoelectric board 101 of the related art 1 is limited to applications that set the piezoelectric board 101 substantially horizontally.
To prevent the slider 105 from dropping off the piezoelectric board 101, a measure is taken in another related art (for example, Japanese Unexamined Patent Application Publication Hei-11(1999)-285279 (page 3, FIG. 1)).
FIG. 2 is a perspective view explaining a surface acoustic wave actuator according to a related art 2.
The surface acoustic wave actuator 200 of the related art 2 shown in FIG. 2 corresponds to the one disclosed in the Japanese Unexamined Patent Application Publication Hei-11(1999)-285279. With reference also to the publication, the surface acoustic wave actuator 200 will briefly be explained.
In FIG. 2, the surface acoustic wave actuator 200 of the related art 2 has a piezoelectric board 201 having a top surface 201a. At left and right ends on the top surface 201a, comb-shaped electrodes 202A and 202B are formed, respectively. When one of the comb-shaped electrodes 202A and 202B receives a voltage from a high-frequency power source (not shown), it generates surface acoustic waves on the piezoelectric board 201.
A mover 203 is set on the piezoelectric board 201 in a path where surface acoustic waves advance. The mover 203 consists of a mover base 203A having many fine protrusions in contact with the top surface 201a of the piezoelectric board 201, a permanent magnet 203B set on the mover base 203A, and a channel-shaped magnetic yoke 203C made of soft magnetic material and serving as a magnetic shield structure for the permanent magnet 203B. These parts 203A to 203C are integrated into one.
The piezoelectric board 201 has an under surface 201b to which a channel-shaped magnetic guide 204 made of soft magnetic material and serving as a linear drive guide is attached in a longitudinal direction (Y-axis direction) of the piezoelectric board 201. On each side of the piezoelectric board 201, the channel-shaped magnetic yoke 203C of the mover 203 and the channel-shaped magnetic guide 204 face each other to produce a magnetic circuit between them. As a result, the mover 203 is preloaded toward the channel-shaped magnetic guide 204 behind the piezoelectric board 201.
When a high-frequency voltage is applied to, for example, the left comb-shaped electrode 202A, surface acoustic waves are generated to serve as a drive source to move the mover 203 in the direction of an arrow mark Y1 along the channel-shaped magnetic guide 204. When the channel-shaped magnetic yoke 203C of the mover 203 and the channel-shaped magnetic guide 204 face each other, magnetic attraction force produced thereby is maximum and stable to return the mover 203 to an original X-axis position even if the mover 203 widthwise deviates from the original X-axis position. If a voltage is applied to the other comb-shaped electrode 202B to generate surface acoustic waves serving as a drive source, the mover 201 advances in an opposite direction from the direction mentioned above.
In this way, the mover 203 is set on the top surface 201a of the piezoelectric board 201, and the linear magnetic guide 204 is attached to the under surface 201b of the piezoelectric board 201. This arrangement allows the piezoelectric board 201 to be inclined, or to be installed upside down, or to be set so that the mover 203 may vertically move.
According to the surface acoustic wave actuator 100 of the related art 1 explained with reference to FIGS. 1A and 1B, the slider 105 may drop off the piezoelectric board 101 if the piezoelectric board 101 is inclined or if it is installed upside down. To prevent this, the surface acoustic wave actuator 200 of the related art 2 shown in FIG. 2 employs some countermeasures. According to the related art 2, the channel-shaped magnetic yoke 203C of the mover 203 and the channel-shaped magnetic guide 204 attached to the under surface 201b of the piezoelectric board 201 produce a closed loop of magnetic flux so that the magnetic flux may hardly leak. Consequently, it is impossible to utilize the magnetic flux that is generated by and moves with the mover 203, for moving another object. The channel-shaped magnetic guide 204 is attached to the under surface 201b of the piezoelectric board 201 in the longitudinal (Y-axis) direction of the piezoelectric board 201, and therefore, the mover 203 arranged on the top surface 201a of the piezoelectric board 201 is movable only in the longitudinal (Y-axis) direction of the piezoelectric board 201. Namely, the mover 203 is unable to two-dimensionally move in X- and/or Y-axis directions. This raises a problem of limiting a range of applications of the surface acoustic wave actuator 200.
This problem will be explained. The surface acoustic wave actuator demonstrates, as mentioned above, excellent working performance such as high speed, high thrust, and high resolution. Due to this, the actuator is applicable not only to a linear motor but also to, for example, a drive source for a two-axis-wobble-type deflector that two-dimensionally wobbles in X- and/or Y-axis directions.
The two-axis-wobble-type deflector is not shown here but an example thereof is shown in Japanese Unexamined Patent Application Publication Hei-6(1994)-180428 that discloses an electrostatic power driven compact optical scanner employing electrostatic drive and in Japanese Unexamined Patent Application Publication Hei-8(1996)-32227 that discloses a planar electromagnetic actuator employing electromagnetic drive. The former employs electrostatic drive, i.e., voltage drive that involves little power consumption because substantially no current flows. The former, however, generates only small force and hardly provides large deflection angles. In addition, the former needs several hundreds of drive voltages.
On the other hand, the latter employing electromagnetic drive of Lorentz force generates large force to provide large deflection angles. In addition, the latter can control the deflection angles by controlling a current value. To maintain a fixed deflection angle, the latter must always supply a fixed current. Accordingly, power consumption will increase when keeping a fixed deflection angle for a certain extent of time.
The surface acoustic wave actuator of the related art 2 is inapplicable to the two-axis-wobble-type deflector because the mover 203 is unable to move two-dimensionally. Namely, the actuator of the related art 2 must be improved if it must conduct two-dimensional movement.